Thomas Stearns Eliot (September 26, 1888 – January 4, 1965) was an Anglo-American poet, playwright, and literary critic, arguably the most important English-language poet of the 20th century. The first poem he became known for, The Love Song of J. Alfred Prufrock, was started in February 1910 and published in Chicago in June 1915, and is regarded as a masterpiece of the modernist movement. It was followed by some of the best-known poems in the English language, including Gerontion (1920), The Waste Land (1922), The Hollow Men (1925), Ash Wednesday (1930), and Four Quartets (1945). He is also known for his seven plays, particularly Murder in the Cathedral (1935).

Born in Saint Louis, Missouri, and educated at Harvard, Eliot studied philosophy at theSorbonne for a year, then won a scholarship to Oxford in 1914, becoming a British citizen when he was 39. "[M]y poetry has obviously more in common with my distinguished contemporaries in America than with anything written in my generation in England," he said of his nationality and its role in his work. "It wouldn't be what it is, and I imagine it wouldn't be so good ... if I'd been born in England, and it wouldn't be what it is if I'd stayed in America. It's a combination of things. But in its sources, in its emotional springs, it comes from America." He was awarded the Nobel Prize in Literature in 1948.

Eliot was born into the Eliot family, a bourgeois family, originally from New England, who had moved to St. Louis, Missouri. His father, Henry Ware Eliot (1843–1919), was a successful businessman, president and treasurer of the Hydraulic-Press Brick Company in St. Louis. His mother, Charlotte Champe Stearns (1843–1929), wrote poems and was a social worker. Eliot was the last of six surviving children; his parents were both 44 years old when he was born. His four sisters were between 11 and 19 years older than him; his brother was eight years older. Known to family and friends as Tom, he was the namesake of his maternal grandfather Thomas Stearns. From 1898 to 1905, Eliot attended Smith Academy, where he studied Latin, Ancient Greek, French, and German. He began to write poetry when he was 14 under the influence of Edward Fitzgerald's Rubaiyat of Omar Khayyam, a translation of the poetry of Omar Khayyam, though he said the results were gloomy and despairing, and he destroyed them. The first poem that he showed anyone was written as a school exercise when he was 15, and was published in the Smith Academy Record, and later in The Harvard Advocate, Harvard University's student magazine.

After graduation, he attended Milton Academy in Massachusetts for a preparatory year, where he met Scofield Thayer, who would later publish The Waste Land. He studied philosophy at Harvard from 1906 to 1909, earning his bachelor's degree after three years, instead of the usual four. Frank Kermode writes that the most important moment of Eliot's undergraduate career was in 1908, when he discovered Arthur Symons's The Symbolist Movement in Poetry (1899). This introduced him to Jules Laforgue, Arthur Rimbaud, and Paul Verlaine, and without Verlaine, Eliot wrote, he might never have heard of Tristan Corbière. He wrote that the book affected the course of his life. The Harvard Advocate published some of his poems, and he became lifelong friends with Conrad Aiken, the American novelist.

He worked as a philosophy assistant at Harvard from 1909–1910, then from 1910–1911, he lived in Paris, studying philosophy at the Sorbonne, where he attended lectures by Henri Bergson and read poetry with Alain-Fournier. From 1911–1914, he was back at Harvard studying Indian philosophy and Sanskrit. He was awarded a scholarship to Merton College, Oxford in 1914. He visited Marburg in Germany first, where he planned to take a summer program, but when the First World War broke out, he went to Oxford instead. There were so many American students at Merton at the time that the Junior Common Room proposed a motion "that this society abhors the Americanization of Oxford," defeated by two votes after Eliot reminded the students how much they owed American culture. But he didn't settle at Merton, and left after a year. He wrote to Conrad Aiken on New Year's Eve 1914: "I hate university towns and university people, who are the same everywhere, with pregnant wives, sprawling children, many books and hideous pictures on the walls ... Oxford is very pretty, but I don't like to be dead." By 1916, he had completed a PhD dissertion for Harvard on Knowledge and Experience in the Philosophy of F.H. Bradley, about F.H. Bradley but he failed to return for the viva voce. In a letter to Aiken late in December 1914, Eliot, aged 26, wrote, "I am very dependent upon women (I mean female society)," and then added a complaint that he was still a virgin. Less than four months later, Thayer introduced Eliot to Vivienne Haigh-Wood, a Cambridge governess. They were married at Hampstead Register Office on June 26, 1915.

After a short, unaccompanied visit to his family in the United States, he returned to London and took several teaching jobs such as lecturing at Birkbeck College, University of London. The philosopher Bertrand Russell took an interest in Eliot's wife while the newlyweds stayed in his flat. Some scholars have suggested that she and Russell had an affair, but the allegations were never confirmed. In a private paper written in his sixties, Eliot confessed: "I came to persuade myself that I was in love with Vivienne simply because I wanted to burn my boats and commit myself to staying in England. And she persuaded herself (also under the influence of [Ezra] Pound) that she would save the poet by keeping him in England. To her, the marriage brought no happiness. To me, it brought the state of mind out of which came The Waste Land." Their relationship was the subject of a 1984 play Tom and Viv, which in 1994 was made into a film. After leaving Merton, Eliot worked as a schoolteacher, most notably at Highgate School, a private school in London, where he taught French and Latin — his students included the young John Betjeman — and later at the Royal Grammar School, High Wycombe, a state school in Buckinghamshire. To earn extra money, he wrote book reviews and lectured at evening extension courses. In 1917, he took a position at Lloyds Bank in London, working on foreign accounts. On a trip to Paris in August 1920, he met the writer James Joyce and artist Wyndham Lewis. Eliot said he found Joyce arrogant — Joyce doubted Eliot's ability as a poet at the time — but the two soon became friends, with Eliot's visiting Joyce whenever he was in Paris. In 1925, he left Lloyds to join the publishing firm Faber and Gwyer, later Faber and Faber, where he remained for the rest of his career, eventually becoming a director. Wyndham Lewis and Eliot became close friends, a friendship leading to Lewis's well-known painting of Eliot in 1938.

On June 29, 1927 Eliot converted to Anglicanism, and in November that year took British citizenship. He became a warden of his parish church, Saint Stephen's, Gloucester Road, London, and a life member of the Society of King Charles the Martyr.

By 1932, Eliot had been contemplating a separation from his wife for some time. When Harvard offered him the Charles Eliot Norton professorship for the 1932-1933 academic year, he accepted and left Vivienne in England. Upon his return, he arranged for a formal separation from her, avoiding all but one meeting with her between his leaving for America in 1932 and her death in 1947. Vivienne was committed to the Northumberland House mental hospital, Stoke Newington, in 1938, and remained there until she died. Although Eliot was still her husband, he never visited her.

From 1946 to 1957, Eliot shared a flat with his friend John Davy Hayward, who gathered and archived Eliot's papers, styling himself "Keeper of the Eliot Archive". Hayward also collected Eliot's pre-Prufrock verse, commercially published after Eliot's death as Poems Written in Early Youth. When Eliot and Hayward separated their household in 1957, Hayward retained his collection of Eliot's papers, which he bequeathed to King's College, Cambridge in 1965.

On January 10, 1957, Eliot married Esmé Valerie Fletcher, 37 years younger than him. In contrast to his first marriage, Eliot knew Fletcher well, as she had been his secretary at Faber and Faber since August 1949. They kept their wedding secret; the ceremony was held in a church at 6.15 a.m. with virtually no one in attendance other than his wife's parents. Since Eliot's death, Valerie has dedicated her time to preserving his legacy; she has edited and annotated The Letters of T. S. Eliot and a facsimile of the draft of The Waste Land. In the early 1960s, by then in failing health, Eliot worked as an editor for the Wesleyan University Press, seeking out new poets in Europe for publication.

Eliot died of emphysema in London on January 4, 1965. For many years he had had health problems caused by his heavy smoking, and had often been laid low with bronchitis or tachycardia. His body was cremated at Golders Green Crematorium. According to Eliot's wishes, the ashes were taken to St Michael's Church in East Coker, the village from which his ancestors had emigrated to America. There, a simple wall plaque commemorates him with a quotation from his poem "East Coker": "In my beginning is my end. In my end is my beginning." On the second anniversary of his death, he was commemorated by the installation of a large stone in the floor of Poets' Corner in London's Westminster Abbey. The stone, cut by designer Reynolds Stone, is inscribed with his dates, his Order of Merit, and a quotation from his poem, "Little Gidding": "the communication / Of the dead is tongued with fire beyond / the language of the living."